Combining submerged membrane technology with anaerobic and ...

Chapter 6Membrane bioreactors (MBR) might be the suitable technology as a post-treatmentfor an anaerobic digester effluent. Methanogenic reactors have been operated as a pretreatmentstep, followed by an aerobic MBR system, for the treatment at environmentaltemperatures of domestic and industrial wastewaters (He et al., 2003; Sánchez et al.,2013). Despite the higher energy consumption referred for this kind of systems, the use ofmembranes would produce a high quality effluent, suitable for reuse.In this sense, an MBR for promoting the removal of nitrogen and dissolved methaneshould consist of a first anoxic chamber, in order to limit methane emissions and promotedenitrification coupled to methane oxidation. Moreover, given that the presence of nitrate inthe effluents of methanogenic reactors is negligible (van Haandel and Lettinga, 1994),nitrate should be recycled from an aerobic chamber in which ammonia is nitrified. The useof MBR systems could be a good choice to enhance denitrification coupled to methaneoxidation as result of the high sludge concentration of these systems, typically between 8-12 gMLVSS·L -1 for submerged MBR systems (Judd, 2011). Denitrification coupled tomethane oxidation is characterised by its low specific denitrification activity. Differentauthors, using batch assays, found activities in between 15 and 90 mgN·gMLVSS -1·d -1 attemperatures around 20-25 ºC (Lee et al., 2001; Khin and Annachhatre, 2004). Thesevalues are much lower than 250 mgN·gMLVSS -1·d -1 referred for denitrification with readilybiodegradable organic matter under similar conditions (Henze et al., 2002). The presenceof biofilms in the anoxic tank would be beneficial for microbial diversity (Shen et al., 2013),assuring that part of the biomass would remain always under anoxic conditions andincreasing the effective biomass concentration in this chamber. Moreover, the installationof a membrane in the aerobic compartment would allow complete microorganismsretention in the system. Kampman et al. (2012) estimated that around 50 % of producedbiomass was washed out from a sequencing batch reactor in which the growth of thenewly discovered anaerobic denitrifying methanotrophic biomass, was promoted.6.2. ObjectivesThe main objective of this research was to study denitrification coupled to methaneoxidation in an MBR using the dissolved methane present in the effluent of an anaerobicUASB system.152